1. Field of the Invention
The present invention relates to a piercing method to be carried out at the start of processing in cutting a workpiece by means of a laser processing machine.
2. Description of the Related Art
In laser processing in which a laser beam is applied to a workpiece for cutting, a piercing operation must be performed at the start of the cutting work. This piercing operation is extremely difficult if the workpiece is a mild steel plate having a thickness of 9 mm or more, in particular. This is because the mild steel plate, which constitutes the workpiece, is susceptible to oxidation by oxygen to be used as an assistant gas, and the oxidation is liable to be excessively promoted by the laser beam, thereby causing an increase of scattered molten metal (sputters) and a thermal runaway. Thus, the piercing operation is carried out while repeating ON and OFF for the supply of the laser beam so that heating by the laser beam and cooling by a jet of the assistant gas alone can be repeated alternately.
According to a conventional piercing method, the piercing operation is performed with the output of the laser beam maintained constant, that is, with the peak value, frequency, and pulse duty ratio of laser pulses maintained constant, from the start to end of the processing. As a method for shortening the piercing time, there is a known method in which the processing time is shortened by increasing the peak value of the laser pulses so that the piercing depth for each cycle of operation can be increased. In order to increase the peak value, the power supply capacity must be increased, which entails higher equipment costs. Moreover, in practical use of such method, the high peak value makes the processing unstable, which ends up with a higher degree of fluctuation of the processing time. Further, the high peak value results in increased sputters, which may adhere to the inside of a processing nozzle, thereby clogging the nozzle, or damaging a focussing lens. Furthermore, it is hard to maintain stable processing over a prolonged period of time, and the peak output, in general, can be maintained only for a very short period of time, so that a resulting hole is too small for the normal cutting work to be started on completion of the piercing operation.
As an alternative method for shortening the piercing time, a method is described in Published Unexamined Japanese Patent Application No. 3-118989, in which the laser output value is lessened at the start of the piercing operation, and is continuously increased with time until an optimum laser output value is reached, and then the piercing operation is continued to the end with this output value. This method, however, is described as requiring 20 to 30 seconds from start to end in piercing a steel plate of 12-mm thickness. Also described in the aforesaid patent application is a method in which both the assistant gas pressure and the laser output value are increased concurrently and continuously, and a method in which the gap between the nozzle tip and the workpiece is narrowed while increasing the laser output value. Even when these methods are used, however, it is believed that the operation for piercing a steel plate of 12-mm thickness requires 15 to 20 seconds.
According to the methods described in Published Unexamined Japanese Patent Application No. 3-118989, control of the operation is very difficult, since the continuous increase of the assistant gas and continuous narrowing the aforesaid gap must be performed simultaneously with control of the continuous increase of the laser output. According to these methods, moreover, the piercing time cannot be shortened to desired extent. For instance, even with the conventional piercing method in which the piercing operation is performed with the laser output maintained constant, a steel plate of 12-mm thickness can be processed in about 20 seconds, proving no special advantage for time saving.
Thus far, there has not been provided any piercing method with which stable processing can be effected without causing any thermal runaway, the processing time for workpieces of the same material and thickness is subject to only minor dispersion, the processing time is shorter, and continuous processing can be enjoyed for a long period of time. This means that the aforementioned conventional methods are not good enough for meeting such requirements from a practical point of view.